The present invention relates to an image processing method and a printer for subjecting multilevel input image data to multilevel dither processing, so as to convert them into image data having a smaller number of gradients.
Hitherto, in an image forming device using a line head, such as a line LED (light emitting diode) head, a line thermal head or a line ink jet head, bi-level images have been formed by printing dots having the same size on a printing paper, with resolution of the head, that is, at intervals along the raster direction of plural LEDs, which are plural printing elements arranged in a line form, in case of the line LED head, at intervals along the raster direction of plural heating resistors, which are plural printing elements arranged in a line form, in case of the line thermal head, or at intervals along the raster direction of plural ink jetting nozzles, which are printing elements arranged in a line form, in case of the line ink jet head.
In such image forming devices, any character image is simply reproduced as an bi-level image corresponding to resolution of the head. Any photographic image is reproduced by halftone processing such as a systematic dither method or an error diffusion method. In such halftone processing, compatibility with maintaining high resolution and reproduction of high contrast gradients is very difficult, and in particular, in systematic dither processing, resolution is not compatible with gradients.
Incidentally, in recent years, as image forming devices having such a line head, devices have been realized wherein multilevel image data are used and a print area in every pixel is modulated so that an image inside every pixel is represented at any one of several gradients (gray scales). FIG. 15 shows a print head 71 in which plural printing elements are arranged in a line form, and the state of dots printed by the print head 71. For simplification, FIG. 15 shows, as an example, a case wherein a pixel is printed at 3 levels containing a white level. By arranging the same as the print head 71 in parallel, for example, 4 or 3 lines, color images in combination of 4 colors, C(cyan), M (magenta), Y (yellow) and K (black), or 3 colors, C , M and Y can be printed.
In such image forming devices making it possible to record multilevel image data, various types of image processings, such as color convert progressing, UCR (under color removal) processing, or .gamma. correction, are carried out. Subsequently, in order to reproduce specific gradients intrinsic to a printer engine section for carrying out actual image printing operation, multilevel halftone processing such as multilevel dither processing using a screen angle, or multilevel error diffusion processing is carried out for each color, and then multilevel image data of several bits per pixel are obtained. Image reproductibity is improved by concentrating a greater deal of information on every pixel.
A multilevel dither processing, which is one of multilevel halftone processings, is roughly classified into 2 types, dependently on a manner of distributing a threshold array extending over respective planes. The one is a method shown in FIG. 17A, in which thresholds are applied to respective plane units, in increasing order. The other is a method shown in FIG. 17B, in which thresholds are applied to any one of pixels to be processed, in increasing order.
For example, a multilevel dither processing in which input image data of 8 bits are converted into images of 4 levels (2 bits) per pixel by the method as shown in FIG. 17A using a (2.times.2) basic dither matrix shown in FIG. 16 is a dither processing for printers making it possible to stably reproduce an image in every independent pixel without being basically influenced by the state of emergence of a dot in any adjacent pixel, for example, ink jet printers. In this processing, resolution is substantially equal to resolution of the printer engine section, and is very high. Dot density is also high, and spatical frequency becomes highest. This processing is an ideal processing in case wherein images are reproduced by area modulation. However, an image is liable to be reproduced in the whole of a pixel by dots having the same or substantially the same sizes and, in consequence, when a gap from an ideal state is generated by influence by printing accuracy or the like, the gap becomes very visually conspicuous as deterioration of image quality.
A multilevel dither processing in which input image data of 8 bits are converted into images of 4 levels (2 bits) per pixel by the method as shown in FIG. 17B using a (2.times.2) basic dither matrix shown in FIG. 16 is a dither processing which is frequently used for printers causing image formation based on independent pixels to be difficult and unstable, with being easily influenced by the state of emergence of a dot in any adjacent pixel, for example, laser printers and thermal printers. In this processing, the resolution of each image is low, and the dot density in the image is also low. The spatical frequency decreases. When the basic threshold array for this dither processing is made into a fattening type, an image called a dot screen dot is formed. Since the resolution is low, slight printing unevenness in every pixel unit is canceled.
Incidentally, concerning the relationship between a print head and printing accuracy about a print position, a print size and the like, for example in case of ink jet printers, the volume or the direction of ink jetted out from their ink jetting nozzles is frequently scattered dependently on the particular ink jetting nozzle. It is possible to suppress the scattering to such a level that any problems do not arise, but in that case manufacturing costs become very high. However, if the problem of the scattering is not solved, in dots from ink jetting nozzles having a large inner diameter or adjacent dots which are very close, their density becomes high and a black stripe arises while in dots from ink jet nozzles having a small inner diameter or adjacent dots which are apart, their density becomes low and unevenness arises, for example, a white line arises. Thus, the image quality deteriorates.
Thus, originally it is ideal that halftone processing is carried out by the multilevel dither processing shown in FIG. 17A. However, this processing has problems that the processing is easily influenced by printing accuracy and that density unevenness and lengthwise lines arise. Thus, it is worth taking it into account that halftone processing is carried out by the multilevel dither processing shown in FIG. 17B and further deterioration such as density unevenness and vertical stripes is prevented as much as possible. In this method, however, resolution drops to a level corresponding to the basic matrix size unit. Thus, in case wherein original resolution is low, pixels themselves become visually conspicuous and the image quality deteriorates.